1. Field of the Invention
The invention relates to an exposure method and apparatus for exposing patterns formed on a mask onto a photosensitive substrate during a photolithographic process for manufacturing, for example, semiconductor devices, liquid crystal displays, imaging devices (such as, for example, a CCD), or thin film magnetic heads. More particularly, the invention relates to an exposure method and apparatus for use in a so-called step-and-scan type exposure process, in which a mask pattern is successively exposed and transferred onto each shot area on a photosensitive substrate while simultaneously moving (scanning) the mask and the substrate in synchronization in predetermined scanning directions.
2. Description of Related Art
Conventionally, a batch exposure type projective exposure apparatus, such as a stepper, has been used to manufacture semiconductor elements or the like using a photolithographic technology. Such apparatus expose images of a pattern on a reticle used as a mask through a projective optical system and onto each shot area on a wafer (or a glass plate) on which photoresist or the like has been applied. Typically, these projective exposure apparatus have used a so-called step-and-repeat method in which the entire pattern from the reticle is exposed onto the wafer shot area (i.e., the area where one chip is formed) in one shot. The wafer is then stepped to the next position, and this process is repeated for the next shot area. This process is repeated until the requisite number of shot areas on the wafer have been exposed. This process typically is repeated many times, using different reticle patterns to build up the various circuits, for example, on the plurality of chips formed on the wafer.
Difficulties have arisen in using the step-and-repeat method due to the trend in the semiconductor industry to produce chips having larger sizes. Due to the larger size chips, the reticle patterns, and the resulting image that is exposed through the projective optical system, also has been made larger. This requires the exposure field of the projective optical system to be made larger, which causes the projective optical system to become more complicated in order to maintain aberrations within an allowable range throughout the entire exposure field. This increases the already high manufacturing cost and causes the projective optical system to become large, which results in the entire body of the apparatus becoming too large.
In order to respond to the demand for increasing the area of the transferred pattern without enlarging the exposure field of the projective optical system, a scanning exposure type of projective exposure apparatus using the so-called step-and-scan method has been developed. The step-and-scan method successively transfers and exposes patterns on a reticle onto each shot area on a wafer by scanning the reticle and the wafer synchronously with respect to the projective optical system so that, at any instant in time, a portion of the pattern of the reticle is projected onto the wafer through the projective optical system.
Semiconductor devices are manufactured by precisely layering a multitude of circuit layers on a wafer (i.e., the substrate). The projective exposure system is provided with stage mechanisms, including a reticle stage and a wafer stage, that move the reticle and the wafer, respectively, to predetermined positions (i.e. positioning), an alignment system, and a movable field stop (a movable blind). As the chips become highly integrated, each of the above mechanisms requires a more precise positioning ability. This typically requires additional time to be spent in order to improve the positioning accuracy. On the other hand, reducing the processing time is also highly desired in order to achieve a high throughput (i.e., a high productivity). Thus, a technique for improving both the positioning preciseness and the productivity is demanded.
In a typical projective exposure system, the reticle pattern is transferred onto a plurality of shot areas one at a time. The exposure system repeats a series of actions, which include moving a target shot area (i.e., the substrate) to be exposed to the vicinity of the exposure field (this is referred to as a stepping action), aligning the shot area to the reticle, and exposing the reticle pattern onto the shot area. In particular, the following series of steps are repeated in a projective exposure system of a scanning exposure (i.e., step-and-scan) type: stepping the wafer stage to the scan start position of the next shot area, accelerating the reticle stage and the wafer stage so as to reach a predetermined scan velocity at or before the exposure start position, adjusting the relative position between the reticle stage and the wafer stage with a high positioning accuracy (i.e., to within a very small tolerance), and driving the reticle stage and the wafer stage at predetermined scan velocities to carry out scanning exposure, while synchronously controlling other driving systems, such as, for example, the movable blind. In such a conventional system, the positioning preciseness (tolerance) at the scan start position and the positioning preciseness (tolerance) during scanning exposure were set to be at substantially the same level.
In the conventional step-and-repeat projective exposure system, the stepping action to the next shot area is by the shortest distance, which is a straight line, and is performed immediately after exposure of a current shot area is completed. Then, positioning (i.e., alignment of the shot area to the reticle) is performed with a high preciseness at the next shot area, and exposure is performed at that position.
On the other hand, in a scanning exposure type (i.e., step-and-scan) projective exposure system, when exposure of the current shot area is finished, the wafer stage steps to a scan start position for the next shot area while decreasing its velocity. The scan start position is spaced a distance from the next shot area so that the wafer stage can accelerate to the desired velocity before reaching the next shot area. After the wafer stage is precisely positioned at the scan start position, scanning exposure is carried out by, for example, reversing the scanning direction. Thus, in the scanning exposure type projective exposure system, after the next shot area of the wafer stage is transversely displaced from the center of exposure of the projective optical system, positioning is performed at the scan start position of the next shot area. For this reason, it is difficult for the scanning exposure type system to have a throughput that is as high as that for the step-and-repeat projective exposure system.